1. Field of the Invention
The present invention relates to a process for producing a magnetic metal powder for use in magnetic recording and particularly to a process for producing a magnetic metal powder having a high specific surface area, a high coersive force, a high dispersibility and excellent corrosion resistance.
Further, the present invention relates to a coating for a magnetic recording medium having excellent corrosion resistance and magnetic characteristics wherein use is made of a ferromagnetic powder.
2. Description of the Related Arts
The recent development of various recording systems is remarkable. Significant among these is the advance in the reduction in size and weight of a magnetic recording/reproducing apparatuses. With this advance, the demand for an increase in the performance of magnetic recording media such as magnetic tapes and magnetic disks has become increased.
A magnetic powder having a high coercive force and a high saturation magnetization is necessary for meeting the above-described demand in magnetic recording. Acicular magnetite and magnetite or the so-called cobalt-containing iron oxide prepared by modifying these magnetic iron oxide powders with cobalt have hitherto been used as a magnetic powder for magnetic recording. A ferromagnetic metal powder having higher coercive force and saturation magnetization, i.e., the so-called magnetic metal powder, has begun to be used for the purpose of producing a magnetic medium having a higher output.
Various processes have been proposed for producing a magnetic metal powder. In general, a process which comprises heating an acicular goethite, or an iron oxide particle prepared by thermally dehydrating the acicular goethite in a reducing gas atmosphere, such as hydrogen, to reduce the goethite to a metallic iron has hitherto been used from the viewpoint of profitability. In this process, however, since the reduction is conducted at a high temperature, fusion of particles, collapse of the shape, etc. tend to occur, so that satisfactory performance can not be obtained. This has led to various proposals. Examples of the proposals include a process which comprises treating goethite with water glass and firing the treated goethite for reduction (see Japanese Patent Publication No. 49722/1988), a process which comprises thermally dehydrating goethite, depositing a silicon compound on the surface of the goethite and heating the goethite for reduction (see Japanese Patent Laid-Open No. 80901/1984) and a process which comprises coating goethite with aluminum phosphate and heating the coated goethite for reduction (see Japanese Patent Laid-Open No. 67705/1988).
However, the problems which can be solved by the above-described proposals are limited to the case where the particle diameter is relatively large, and these proposals are unsatisfactory in the case of a particulate magnetic metal powder having a high specific surface area close to 60 m.sup.2 /g to cope with high-density recording in recent years.
Further the particulate magnetic metal powder has another problem. Specifically, the magnetic metal powder is chemically unstable and susceptible to a reduction in the saturation magnetization with time unfavorably. The smaller the particle, the more significant this drawback. Various proposals have been made for the purpose of solving this drawback. The present inventors have found that the formation of a surface transition metal layer contributes to an improvement in the oxidation resistance, and already filed patent applications (see Japanese Patent Laid-Open Nos. 62205/1988 and 64006/1989). The above-described methods of approach to the problems exhibit a significant effect when the particle diameter is relatively large. In the case of a fine particle, although they bring about oxidation resistance superior to that of the case where no treatment is conducted, the effect attained is not satisfactory yet.
An object of the present invention is to prevent the occurrence of sintering of particles with each other in the step of producing a particulate magnetic metal powder and to provide a process for producing a magnetic metal powder having excellent magnetic characteristics and oxidation resistance and a coating for a magnetic recording medium wherein use is made of said magnetic metal powder.